The present invention relates to optical spectrum analyzers, such as are used in optical multiplexers and demultiplexers, and, more particularly, to a monolithic optical spectrum analyzer based on a volume phase grating.
Fiber-optic systems are presently being developed for high-bandwidth, high-speed voice, video, and data communications. Originally, single-channel systems in which each fiber carried a single channel sufficed, but increasing traffic has led to a need for greater bandwidth. Because of the high cost of laying optical fibers, achieving greater efficiency in utilizing existing fiber resources has become increasingly important. This trend has led to the development of systems which provide two-way multiple channel communications over a single fiber.
In fiber optic systems, a convenient way to carry multiple channels over a single fiber without interference between the channels, for both one-way and bi-directional communications, is by means of a technique known as wavelength division multiplexing (WDM). According to this technique, multiple signals of separate wavelengths are coupled to the fiber at the transmitting side and separated at the receiving side. WDM is now a well-established technology to increase the information capacity of fiberoptic links, with 4, 8, and 16 channel systems already installed. These systems typically use cascaded discrete components such as interference filters or fiber gratings to separate channels with spacing of 200 to 400 GHz. The next generation of WDM systems expected to be deployed in the near future is Dense Wavelength Division Multiplexing and will support up to 60 or more 100 GHz channels or up to 120 or more 50 GHz channels. For such high channel counts, a parallel architecture is required for multiplexing/demultiplexing units because cascading a large number of systems can lead to problems including cumulative crosstalk, center wavelength inaccuracy, polarization dependent wavelength shift, and temperature instability.
Currently, there are two approaches that are capable of meeting these requirements: phased array waveguides, and diffraction gratings. The former is a monolithic approach that has been reported extensively in the literature but has significant problems for very large channel counts and requires temperature control. The latter approach with diffraction gratings has also been reported extensively in the literature. Most of the reported techniques utilize surface-relief diffraction gratings. In recent years, a new type of grating has been developed using holographic techniques, called the volume phase (VP) grating. Rather than being diffracted by surface-relief structures as in a surface grating (typically generated either by diamond ruling or by a holographic exposure of a photoresist layer), in a VP grating the light undergoes Bragg diffraction as it passes through the volume of a thin layer of material, in which the refractive index is modulated (for example, hardened dichromated gelatin). These volume-phase holographic gratings show improved performance over classical, low-order surface-relief gratings with respect to the following performance characteristics:
1. The blaze or efficiency envelope is governed by Bragg diffraction and can be tuned by adjustment of the grating angle for different wavelengths or diffraction orders.
2. VP gratings can have high diffraction efficiencies, approaching 100% for high line density (600 to 6000 l/mm), high dispersion transmission gratings, with low dependence on polarization angle.
3. Complex structures including the grating itself and collimating and focusing optics can be produced, in a monolithic configuration, to minimize the required number and/or complexity of optical elements, simplifying optical design and alignment and enhancing device efficiency.
4. The grating can be sandwiched between two appropriate substrates, to provide an environmentally stable device which is robust and has long lifetime without degradation.
A few prior art systems, based on the VP grating, have been shown to be suitable for multiplexing and demultiplexing. See, for example, Behzad Moslehi et al., xe2x80x9cFiber-optic wavelength-division multiplexing and demultiplexing using volume holographic gratingsxe2x80x9d, Optics Letters vol. 14 no. 19 pp. 1088-1090 (1989) and Charles C. Zhou et al., xe2x80x9cAxial-graded-index (AGRIN) lens-based eight-channel wavelength division demultiplexer for multimode fiber-optic systemsxe2x80x9d, IEEE Photonics Technology Letters vol. 10 no. 4 pp. 564-566 (1998).
The device reported by Zhou et al. utilizes multimode fibers of 50 micrometer core diameter, 500 GHz spacing, covers a wavelength range from 764 to 792 nm, and utilizes Gradient Index Lenses (GRIN) as the imaging optics. State-of-the-art GRIN imaging optics are available commercially only in relatively small diameters, typically 0.5 to 3 mm. In fact, only very small diameter GRIN lenses can be manufactured with high precision and repeatability, as far as their doping profile is concerned, which in turn determines the index of refraction profile. As a result, the required high resolution of 50-100 GHz can be achieved only in two ways, either by using large surface gratings or by using large diffraction angles or grazing angles. The latter configuration, based on large diffraction angles, decreases the grating efficiency due to losses in the cavity. On the other hand, the former method, based on large surface gratings, requires larger imaging optics, so that an area of at least 1 centimeter width of grating is illuminated without degradation of efficiency. Because GRIN lenses can not be manufactured practically with diameters greater than about 3 millimeters, GRIN lenses are not optimal for this application.
The device of Moslehi et al. uses a VP grating and operates in the wavelength range of 710 to 900 nm and in a Littrow configuration. This is done by tilting the grating planes at an angle almost parallel with the grating surface. When the device is used in a reflection configuration the grating efficiency can be quite high for low dispersion applications (larger than 15 nm separation); however, for high dispersion applications, such as those required for DWDM (wavelength spacing of 0.8 nm to 0.4 nm) the efficiency bandwidth is narrower than desired.
Zhang et al., in U.S. Pat. No. 6,108,471, teach a wavelength demultilplexer in which a lens collimates a multi-wavelength input beam from an input optical fiber onto a VP grating. The resulting diffracted beams are reflected back through the VP grating to be diffracted a second time, and then are focused by the lens onto their respective output fibers.
The device of Zhou et al. is monolithic. The devices of Moslehi et al. and Zhang et al. are made of discrete components, and so are not as mechanically stable as the device of Zhou et al.
Kaiser Optical Systems of Ann Arbor Mich. offers for sale the monolithic, VP-grating-based device illustrated in FIG. 1. This device consists of a VP grating 20 sandwiched between two prisms 22 and 24. Also shown in FIG. 1 is a collimated input beam 26 that includes three wavelengths that emerge from the device as three output beams 28a, 28b and 28c. The device of FIG. 1 diffracts input beam 26 at large diffraction angles, and so suffers from the same deficiencies as the Zhou et al. device discussed above.
There is thus a widely recognized need for, and it would be highly advantageous to have, a volume-phase-grating based multiplexer and demultiplexer that would overcome the disadvantages of presently known devices as described above.
According to the present invention there is provided an optical spectrum analyzer including: (a)at least one volume phase grating; (b) a first reflector; and (c) a second reflector; the first and second reflectors being in a fixed spatial relationship with the at least one volume phase grating, so that light entering the optical spectrum analyzer is reflected by the first reflector towards the at least one volume phase grating, the reflected light is diffracted towards the second reflector while traversing the at least one volume phase grating, and the once-diffracted light is reflected back by the second reflector towards the at least one volume phase grating and is again diffracted towards the first reflector while again traversing the at least one volume phase grating.
According to the present invention there is provided an optical spectrum analyzer, including: (a) at least one volume phase grating for diffracting input light; and (b) a first reflector for receiving the diffracted input light from the at least one volume phase grating along a first optical path and reflecting the diffracted input light back to the at least one volume phase grating along a second optical path distinct from the first optical path.
According to the present invention there is provided a method of demultiplexing input light of a plurality of free space wavelengths, including the steps of: (a) diffracting the input light towards a reflector, the input light of each wavelength being diffracted towards the reflector along a respective first optical path; (b) reflecting the diffracted light, using the reflector, the diffracted light of each wavelength being reflected along a respective second optical path distinct from the respective first optical path; and (c) diffracting the reflected light.
According to the present invention there is provided a method of multiplexing input light of a plurality of wavelengths, including the steps of: (a) directing the input light towards a diffraction mechanism; (b) diffracting the input light towards a first reflector, using the diffraction mechanism, the input light of each wavelength being diffracted towards the first reflector along a respective first optical path; (c) reflecting the diffracted light, using the first reflector, the diffracted light of each wavelength being reflected along a respective second optical path distinct from the respective first optical path; and (d) diffracting the reflected light; the directing of the input light being effected so that all the twice-diffracted light travels along a common optical path.
As understood herein, an optical channel is a beam of light, of a certain carrier wavelength, that is modulated to carry signals. Unless otherwise specified, the wavelength of an optical channel is understood herein to be the free space carrier wavelength of the optical channel.
As understood herein, an xe2x80x9coptical pathxe2x80x9d is a region in space through which a beam of light propagates. The beam of light may be converging, diverging or collimated.
As understood herein, xe2x80x9cmultiplexingxe2x80x9d is the operation of merging several optical channels, of different carrier wavelengths, that initially travel along separate respective optical paths, to travel along a common optical path. xe2x80x9cDemultiplexingxe2x80x9d is the inverse of multiplexing: the separation of several multiplexed optical channels to travel along separate respective optical paths. In optical communications, the optical paths typically are defined by waveguides such as optical fibers.
The device of the present invention is a modular optical spectrum analyzer. Like the prior art device of FIG. 1, the device of the present invention is based on a VP grating sandwiched between two transparent blocks. Unlike the prior art device of FIG. 1, the transparent blocks of the present invention are provided with reflective surfaces that enable the double diffraction of an input beam (for demultiplexing) or of several input beams (for multiplexing) at low diffraction angles. The diffraction angles of the present invention are preferably less than 23 degrees, and most preferably less than 16 degrees. The first block includes a portal surface through which light enters and exits the device, and a first, preferably curved, most preferably paraboloidal or spherical, reflective surface that collimates input light onto the VP grating. The second block includes a second reflective surface for reflecting the once-diffracted light back to the VP grating to be diffracted a second time. Although the second surface may be planar, preferably, the second reflective surface is a dihedron, so that the once-diffracted light travels to and from the second reflective surface along two distinct optical paths.
For multiplexing, one output optical fiber and several input optical fibers are optically coupled to the first block at the portal surface. For demultiplexing, one input optical fiber and several output optical fibers are optically coupled to the portal surface. The first reflector, in addition to directing input light towards the VP grating, also directs twice-diffracted light towards the portal surface.
The low diffraction angles of the present invention enable the location of the exit points, on the portal surface, of demultiplexed light, to be a linear function of the carrier wavelengths. The appropriate figure of merit for this linearity is the second derivative of exit point location with respect to carrier wavelength. Preferably, for demultiplexed light having wavelengths between about 1.4 microns and about 1.6 microns, this figure of merit is at most 6xc3x9710xe2x88x929 nanometers per square micron.
To demultiplex a set of optical channels, the optical channels are introduced to the device of the present invention via a single input optical fiber that is optically coupled to the portal surface. The input light is reflected and collimated by the first reflective surface towards the VP grating, diffracted a first time by the VP grating towards the second reflective surface, reflected by the second reflective surface back towards the VP grating, diffracted a second time by the VP grating towards the first reflective surface, and reflected by the first reflective surface towards a set of output optical fibers that are optically coupled to the portal surface, with each channel being diffracted towards its respective output optical fiber.
To multiplex a set of optical channels, the optical channels are introduced to the device of the present invention via respective input optical fibers that are optically coupled to the portal surface. The input light is reflected and collimated by the first reflective surface towards the VP grating, diffracted a first time by the VP grating towards the second reflective surface, reflected by the second reflective surface back towards the VP grating, diffracted a second time by the VP grating towards the first reflective surface, and reflected by the first reflective surface towards a single output optical fiber that is optically coupled to the portal surface.